In accordance with an embodiment, described herein is a system and method for merging a mainframe data file to a target table in a database used by a mainframe rehosting platform. A plurality of programs can be generated by the mainframe rehosting platform based on the target table and the mainframe data file. The generated programs can be used to create in the database an empty temporary table that has the same structure as that of the target table, to upload the mainframe data file to the empty temporary table, and to use an existing merge function in the database to merge the temporary table with the target table. When uploading the mainframe data file to the temporary table, records in the mainframe data file can be read and stored into an array, which can be inserted into the temporary table in a single insert operation.